Large avatar images hanging around in blogs.dir

I noticed that my WPMU/BuddyPress installation was starting to grow rapidly and pretty quickly tracked down the source: avatar images. It seems that there are some very large (1MB+) images in the avatar folders below blogs.dir. In most of the folders there are just two small images (the avatars at two different sizes), but in some cases there are other large images.

Some experimentation confirmed that this can happen when people download an image for their avatar, but don’t complete the process for changing the avatar image.

Is there anything built into BP to clean this up, or does one have to go in there periodically and do it manually?